2,5,5-Trimethyl-3-carbalkoxy-4(β,β-dihalovinyl)-4,5-dihydrofurans

ABSTRACT

A method of preparing certain dihalogen vinyl cyclopropane carboxylic acid esters of the formula ##STR1## wherein R is an alkyl moiety and X is chlorine or bromine by exposing a dihalogen vinyl dihydrofuran of the formula ##STR2## to light, e.g., ultraviolet light. Also disclosed is a method for preparing such 2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-dihydrofurans by reaction of a β-alkoxycrotonic acid ester or 3,3-bisalkoxybutyric acid ester with a 1,1,1-trihalogen-4-methyl-3- or -4-pentene-2-ol in the presence of an acid catalyst. Also disclosed is a method of converting such 2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-dihydrofuran by thermal rearrangement into 2,5,5-trimethyl-3-carbalkoxy-4-(β,β-dihalogenvinyl)-4,5-dihydrofurans. Such 2,4,4- and 2,5,5-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-dihydrofurans are new.

This is a division of application Ser. No. 937,622, filed Aug. 28, 1978.

BACKGROUND

The subject of the invention is a method of preparing dihalogenvinylcyclopropanecarboxylic acid esters of the general formula ##STR3##in which R¹ is a straight-chain or branched alkyl moiety and Xrepresents chlorine or bromine.

The moiety R¹ is, for example, an alkyl moiety having 1 to 18,preferably 1 to 8 carbon atoms, especially 1 to 2 carbon atoms.

Known methods of preparing compounds of Formula I are described inGerman Offenlegungsschrift Nos. 2,606,635 and 2,649,856. Both methodsset out from a 3,3-dimethylbutene-1 bifunctionalized in position 4, fromwhich compounds of Formula I are obtained by adding on CCl₄, forexample, and then cyclizing with bases. In both methods, staringmaterials are used which are difficult to obtain, so that the entireprocess is uneconomical and difficult.

THE INVENTION

It is the object of the present invention to provide a method for thepreparation of cyclopropanes of General Formula I which will begenerally applicable and economical on a commercial scale.

This object is achieved in accordance with the invention by transforminga dihalogen vinyl dihydrofuran of the general formula ##STR4## in whichR1 and X have the same meaning as above, to compounds of Formula I byexposure to light.

The three-ring structure of Formula I, therefore, is obtained by aphotochemical ring contraction of the correspondingly substituteddihydrofuran compound II.

It has surprisingly been found that very good yields can be achieved bythe method of the invention. Contrary to expectations, only smallamounts of undesired monomers and polymers are formed.

The ring contraction is performed with especially good yields byexposure to light having a wavelength greater than 200 nm. Especiallysuitable wavelength ranges are between 200 and about 400 nm, preferablybetween 230 and 300 nm.

The light source is preferably an ultraviolet lamp. However, any otherlight source which produces ultraviolet light or light containingultraviolet can be used.

The reaction can be performed in the liquid phase, exposing, forexample, the compound of Formula II as it is.

It is preferably to operate in the presence of an inert solvent, usingsolvents which are transparent to the radiation and do not enter intoany reaction with the compounds of Formula I or of Formula II. Examplesof suitable solvents are acetonitrile, alcohols, especially lowaliphatic alcohols, diethyl ether, dioxane and the like, individually orin mixtures. Preferred as solvents are the low aliphatic alcohols,preferably methanol or ethanol.

The irradiation can be performed either by means of a lamp immersed inthe liquid to be treated or by means of a source or several sourcesdisposed on the outside of a reactor transparent to the radiation.Another variant consists in circulating the solution through the lightexposure zone in a thin layer, such as a film for example, by means ofcirculation pumps. The liquid can also be subjected to illumination inthe form of fine droplets, e.g., in the form of an emulsion or by theuse of a spraying means. It is desirable to use inert gases, such asnitrogen and/or noble gases as the spraying aid.

In many cases, it is desirable to perform the irradiation in the absenceof oxygen or gases containing oxygen. Inert gases of the above-describedkind can be used for purging the liquid being exposed as well as thereactors, and as a shielding gas during the exposure.

If a submerged lamp is used or if the exposure of a liquid contained ina reactor is performed by means of a light source such as a lamp, forexample, disposed outside of the reactor, it is desirable to keep theliquid in constant movement, by means, for example, of a stirrer or bymeans of an inert gas introduced into the liquid.

The ratio of admixture of the compounds of Formula II with the solventcan vary widely. Preferentially, 5x molar to 0.001x molar solutions areused for the irradiation.

The irradiation can also be performed in the gaseous phase, e.g., bymeans of a submerged lamp or a light source outside of the reactor, itbeing desirable to irradiate the gaseous starting product of Formula II,which can be mixed if desired with a gaseous solvent of theabove-described kind, with the exclusion of gases containing oxygen, ifdesired. It is expedient to operate in the presence of an inert gas,such as nitrogen, and/or noble gases such as argon or helium.

Basically, one can also operate in the solid phase, although irradiationin the liquid phase or gaseous phase is preferred for commercialreasons.

The irradiation of the invention can be performed over a very broadrange of temperatures. When operating in the liquid phase and gaseousphase, the limits are defined by the phase transformation points of theliquid or gas. The upper temperature limit is determined by the amountof thermal stress which the compounds of Formulas I and II as well asany solvents used can withstand.

In general, the reaction takes place satisfactorily in a temperaturerange between -50° and +250° C. A preferred temperature range is between-20° and +150° C., preferably 0° C. to 100° C. Since the reaction takesplace without change in volume, it is independent of pressure. It issimplest, therefore, to operate at standard pressure; nevertheless,elevated or reduced pressure can also be applied.

In the reaction in the gaseous phase, however, it is preferable to applya vacuum for the purpose of converting the liquid to the gaseous phase.

The process of the invention can be performed continuously ordiscontinuously.

The light exposure time depends on the flux density of the light sourceand on the amount of starting product involved, among other things. Theoptimum time can be determined by analytic methods, e.g., by gaschromatography.

The2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-di-hydrofuransof Formula II are new. They can be obtained by the reaction of at leastone β-alkoxycrotonic acid ester of the general formula ##STR5## and/orat least one 3,3-bisalkoxybutyric acid ester of the general formula##STR6## wherein R¹, R², R³ and R⁴ are alike or unalike, preferablyalike, and are straight-chain or branched alkyl moieties, with (a) atrihalogen-4-methyl-3-pentene-2-ol of the general formula ##STR7## inwhich X represents chlorine or bromine atoms and/or with (b) atrihalogen-4-methyl-4-pentene-2-ol of the general formula ##STR8## inwhich X represents chlorine atoms or bromine atoms, in the presence ofan acid catalyst.

The moieties R¹, R², R³ and R⁴ are alkyl moieties of 1 to 18, preferably1 to 8, and especially 1 or 2 carbon atoms.

Examples of suitable catalysts are Lewis acids, such as AlCl₃, ZnCl₂,BF₃, FeCl₃ and the like, protonic acids (Bronsted acids) such as H₂ SO₄,KHSO₄, KH₂ PO₄ H₃ PO₄, p-toluenesulfonic acid, trichloroacetic acid, andacid ion exchangers. Mixtures of the individual acids can also be used.

The catalysts or catalyst mixtures are employed in amounts of up to 20mole-%, preferably of less than 10 mole-%, with respect to the startingsubstance of General Formulas III and/or IV.

In the method of the invention for the preparation of the compounds ofFormula II, hydrogen halide and an alcohol of the formula R² --OH areliberated, provided that R³ and, in some cases, R⁴ are the same as R².If they are not, corresponding alcohol mixtures are liberated. Bothproducts are preferably removed during the reaction.

The reaction is preferably performed at standard pressure, but in somecases with the application of a vacuum, for example for the distillativeseparation of a higher alcohol (higher-boiling alcohol) formed duringthe reaction, such as, for example, an alcohol having a boiling pointhigher than 160° C. at 760 mm Hg.

The compounds of Formula II and/or IV are reacted with the compounds ofFormulas V and/or VI in a molar ratio of 1:0.5 to 1:10. Preferably,compounds V and/or VI are used in an excess. Preferably, the compoundsof Formula III and/or IV are reacted with one of the compounds ofFormulas V or VI, the radicals R¹, R², R³ and R⁴ being preferably alike.

The reaction is performed in a substantially water-free medium. It istherefore desirable to use starting substances which are as dry aspossible, and to conduct the reaction with the substantial exclusion ofmoisture.

The reaction can be performed in the presence of an inert solvent withthe substantial exclusion of moisture. Inert solvents are, for example,those which do not render the starting product and the catalyst uselessfor the reaction and which do not react chemically with the end product.Suitable solvents are, for example, chlorobenzenes such as xylenes,dioxane, and dimethylformamide.

In general, amounts of solvents are used which will result, for example,in ratios of Compound III and/or IV to the solvent of from 2:1 to 1:10.Basically, however, the ratio can be greater than 2:1 or less than 1:10.

The reaction can, in principle, also be performed without the use of asolvent.

The reaction is performed at elevated temperature. In general,temperatures of 80° to 200° C. are employed, preferably 120° to 160° C.

Preferably the reaction is performed at the temperature at which thealcohol R² --OH that forms or the alcohol mixture that forms can beseparated by distillation at standard pressure or, in some cases, atreduced pressure, without doing thermal damage to the starting and endproducts.

The mixture should be stirred constantly during the reaction.

The process is generally performed in the following manner: Thecompounds of Formulas III and/or IV are heated with the trihalogencompounds of Formulas V and/or VI, with stirring, in the presence of thecatalyst. The alcohol that forms is removed by distillation during thereaction. Preferably compounds III and/or IV are fed in portions orcontinuously to compounds V or VI or to a mixture of compounds V and VI.It is desirable that the feeding of these compounds be extended over theentire reaction time of, for example, 2 to 12 hours. After the reactionhas ended, the mixture is worked up by fractional distillation in vacuo.

In the first fraction one obtains the starting compound V and/or VIwhich was used in excess, and in the second fraction the dihydrofuran.

The compounds used in excess can be recycled without furtherpurification after they have been separated by distillation.

For the preparation of the dihalogen vinyl cyclopropanocarboxylic acidester of General Formula I, the reaction product which forms immediatelywhen the bisalkoxybutyric acid ester and/or alkoxycrotonic acid ester isheated with at least one of the trihalogen compounds of Formulas V or VIwhile the alcohols that form are constantly removed by distillation, canbe exposed to light in accordance with the invention, preferably afterremoval of the catalyst. Preferably, however, the reaction product isfractionally distilled to remove excess starting compounds before it issubjected to exposure in accordance with the invention. The same appliesto the preparation of the2,5,5-trimethyl-3-carbalkoxy-4-(β,βdihalogenvinyl)-4,5-dihydrofurans(VII) by the thermal rearrangement of2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-dihydrofurans(II).

The 3,3-bisalkoxybutyric acid alkyl esters (IV) are, for example, thoseobtainable by the method described on page 1199 of Beilstein, E III 3.

The β-alkoxycrotonic acid esters (III) can be prepared by reactingacetic acid ester with a mixture of alcohol and an orthoformiate (A.Michael, G. H. Carlson, J. Amer. Chem. Soc. 57 (1935), page 162).

The trihalogen compounds of Formulas V and VI are obtainable by thereaction of isobutylene with trihalogen acetaldehydes (J. Colonge, A.Perrot, Comptes Rendues 239 (1954), page 541; E. J. Klimova, Chem.Abstr. 71 (1969 112335 K).

Additional subject matter of the invention are2,5,5-trimethyl-3-carbalkoxy-4-(β,β-dihalogenvinyl)-4,5-dihydrofurans ofthe general formula ##STR9## in which R¹ represents a straight-chain orbranched alkyl radical and X represents Cl or Br, and a method ofpreparing dihydrofurans of Formula VII.

R¹ can be an alkyl moiety of 1 to 18 carbon atoms, preferably of 1 to 8carbon atoms, and especially of 1 to 2 carbon atoms.

2,5,5-Trimethyl-3-carbalkoxy-4-(β,β-dihalogenvinyl)-4,5-dihydrofuranshave not been known heretofore.

The2,5,5-trimethyl-3-carbalkoxy-4-(β,β-dihalogenvinyl)-4,5-dihydrofuransare obtained by thermally rearranging2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-dihydrofurans ofGeneral Formula II ##STR10## in which R¹ and X have the same meaning asabove.

The thermolysis of the compounds of Formula II is best performed in thegaseous phase. If desired, a mixture of a compound of Formula II and aninert solvent can be subjected to thermolysis. It is recommendable touse those solvents which are in the gaseous state at the temperature ofthe thermolysis.

Fundamentally, the thermolysis can also be performed in the liquidphase, in the presence, if desired, of an inert solvent.

Suitable solvents are aromatic hydrocarbons such as, for example,benzene, or halogenated hydrocarbons such as carbon tetrachloride, forexample.

The thermolysis temperature generally ranges between 200° and 500° C.,although higher temperatures can be chosen if the time of thermolysis isshortened.

The molar ratio of admixture of solvents to compounds of Formula II canbe varied widely. Good results are achieved, for example, by usingapproximately 1 to 1000 moles of the solvent or solvent mixtures permole of the compound of Formula II. Larger amounts of solvent, however,can also be employed.

To facilitate the evaporation of the liquid or the thermolysis, as thecase may be, the liquid is subjected to the heat treatment in the formof a thin layer or fine droplets.

For this purpose it is possible to use, for example, atemperature-controlled reaction tube in which the surface area isenlarged by the use of packing. The tube, in an upright position, forexample, is provided at the top with an opening for the introduction ofthe starting product, and with a means for the introduction of an inertgas.

In the thermolysis, the procedure can be, for example, to feed thecompound of Formula II, dissolved in a solvent if desired, drop by dropin liquid form, or to inject it in the form of a spray using an inertgas as the pressure and tranport medium. The compound to be thermolyzed,mixed, if desired, with an inert solvent, can also be introducedcontinuously or periodically in gas form into the actual thermolysisapparatus.

The rate of flow of the liquid or gas stream and hence the thermolysistime can be regulated on the basis of the dimensions of the reactor, thedepth of fill, the dimensions and design of the packing, and thetemperature of the reaction zone, by adjusting the required pressuregradient between the inlet and outlet openings of the tube and by therate of feed. It is best to determine the optimum conditions bypreliminary experiments.

After passing through the hot reaction zone the thermolyzate, which isin the form of a gas, let us say, is cooled to, for example, roomtemperature. The condensate is then subjected to a fractionaldistillation, preferably with the application of a vacuum.

The process of the invention is performed, for example, in a quartzreactor. Other reactor materials, however, are also suitable, such asceramic material for example. Suitable packing bodies are, for example,bodies of quartz, or also of other materials such as ceramic.

The thermolysis can be performed at standard pressure or higher or lowerpressure. Generally, it can be performed at a pressure of 0.1 Torr to 2atmospheres.

It can be performed either continuously or discontinuously.

It is recommendable to assure the substantial exclusion of oxygen duringthe thermolysis, and for this reason it is desirable to purge thethermolysis apparatus prior to the thermolysis, with an inert gas, suchas nitrogen, or with noble gases such as argon and/or helium. It isdesirable that an inert gas be present also during the thermolysis.

The cyclopropanecarboxylic acid esters of General Formula I are aforeproduct of the acid component of synthetic pyrethroids, which haveacquired industrial importance as insecticides of good persistence andlow toxity to suckling animals (cf. M. Elliott, "Synthetic Pyrethroids,"ACS Symposium, Series 42, American Chemical Society 1977). The activeInsecticide can be synthezised as described in examples 1 and 13 of thisinvention.

The other new compounds of General Formulas II and VII are intermediatesfor the synthesis of insecticides. They can be transformed to compoundsof formula I and further to the active insecticide.

EXAMPLES

The invention will be further explained with the aid of the followingexamples.

EXAMPLE 1

Four grams of2,4,4-trimethyl-3-carbomethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuranare dissolved in 100 ml of acetonitrile and exposed at 0° C. toultraviolet light of a wavelength longer than 200 nm produced by asubmerged lamp. The time of exposure was one hour (lamp: Hanau Q 81,water-cooled medium-pressure mercury bulb). The reactor is a verticallydisposed, temperature-controlled, cylindrical glass reactor having aninside diameter of 5.6 cm and a length of 20 cm. The bulb is housed inan immersion tube of quartz glass through which coolant (water) iscirculated. The bulb and immersion tube are placed in the glass reactorso as to provide approximately 0.5 cm of annular clearance around them.

The contents of the reactor were purged with nitrogen for ten minutesbefore the illumination began, and during the illumination the liquidwas kept in constant movement by the introduction of nitrogen.

The end point of the reaction was determined by gas chromatography.

After the exposure to light, the aoetonitrile is removed in vacuo andthe residue is subjected to vacuum distillation. A uniform fraction wasobtained at 99° to 102° C. (0.1 mm) (3.7 g=92.5% yield) which wasidentified as1-acetyl-2,2-dimethyl-3-(β,β-dichlorovinyl)-cyclopropanecarboxylic acidmethyl ester (A).

MNR spectrum (100 MHz, CCl₄)δ=5.80-6.00 (d d, 1 H); 3.75 (s, 3 H);2.50-2.68 (d, d, 1 H); 2.20 (b s, 3 H); 1.14-1.24 (m, 6 H).

By the deacylation of A by the method described in GermanOffenlegungsschrift No. 2,649,856, the disclosure of which is herebyincorporated herein by reference, page 22, Example 17, a mixture of cisand trans isomers of2,2-dimethyl-3-(β,β-dichlorovinyl)-cyclopropanecarboxylic acid methylester is obtained, which is identical in its spectroscopic data with asample of the genuine substance. This deacylation comprises: Analoguesto example 17 (DE-OS No. 2,649,856) the methylester is treated withNa-methylate. The deacrylated methylester is isolated anologues toexample 17 (DE-OS No. 2,649,856). Analogues to Fankas (Chem. L. sty52688 (1958) the active insecticide can be synthesized(Methylester→acid→acid chloride→reaction with an alcohol e.g.allylmethylene (C.A. Vol. 52, U3650e). The higher allylesters can betreated in an analogues manner.

EXAMPLE 2

The same experimental arrangement was used as in Example 1.

5 g of2,4,4-trimethyl-3-carbethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran wasdissolved in 100 ml of acetonitrile and irradiated at 20° C. withultraviolet light of a wavelength greater than 200 nm. At the end of theirradiation the mixture was worked up as in Example 1. At 98° to 103° C.(0.05 mm), a uniform product passed over (4.7 g=94%), which wasidentified as1-acetyl-2,2-dimethyl-3-(β,β-dichlorovinyl)-cyclopropanecarboxylic acidethyl ester. The structure was proven as in Example 1 by the NMRspectrum and by deacylation.

EXAMPLE 32,4,4-trimethyl-3-carbethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran

242 g of 1,1,1-trichloro-4-methyl-3-pentene-2-ol was mixed with one gramof AlCl₃ and the mixture was heated at 145° C. With the exclusion ofmoisture, 109.5 g of β-ethoxycrotonic acid ethyl ester was added drop bydrop with stirring over a period of 6 hours, while maintaining thetemperature at 145° C. The low-boiling components were distilled outduring the reaction. At the end of the reaction the mixture was workedup by fractional distillation. In the first fraction excesstrichloro-4-methyl-3-pentene-2-ol is obtained. The dihydrofuran passesover in the second fraction (b.p.₀.1 mm 108°-109° C.), yield 177.4 g(91.7%). NMR spectrum (CCl₄, δ): 1.05-1.20 (m, 9 H); 2.10 (s, 3 H);4.0-4.30 (q, 2 H); 4.82 (d, 1 H); 5.97 (d, 1 H).

EXAMPLE 4

6.3 g of 1,1,1-trichloro-4-methyl-4-pentene-2-ol was mixed with 4.7 g ofβ-ethoxycrotonic acid ethyl ester and 0.1 g of AlCl₃ and the mixture isheated for 6 hours at 155° C., while the low-boiling components thatformed were removed by distillation. The high-boiling components arefractionally distilled. 7.2 g is obtained (yield 86%) of2,4,4-trimethyl-3-carbethoxy-5-(β,β-dichlorovinyl)-4.5-dihydrofuran.

EXAMPLE 52,4,4-trimethyl-3-carbomethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran

10.0 g of 1,1,1-trichloro-4-methyl-3-pentene-2-ol was mixed with 0.2 gof AlCl₃ and heated at 150° C. With the exclusion of moisture, 3.6 g ofβ-methoxycrotonic acid methyl ester was added drop by drop, withstirring, over a period of 4 hours, while maintaining the temperature at150° C. The low-boiling substances that formed were removed bydistillation. At the end of the reaction, the mixture was fractionallydistilled. 6.6 g (yield 90.2%) of dihydrofuran was obtained (b.p.₀.1 mm102°-104° C.). NMR spectrum (CCl₄, δ): 1.14 (s, 3 H); 1.24 (s, 3 H);2.15 (s, 3 H); 3.68 (s, 3 H); 4.85 (d, 1 H); 6.0 (d, 1 H).

EXAMPLE 62,4,4-trimethyl-3-carbethoxy-5-(β,β-dibromovinyl)-4,5-dihydrofuran

12 g of 1,1,1-tribromo-4-methyl-3-pentene-2-ol was mixed with 0.1 g ofKHSO₄ and 3.16 g of β-ethoxycrotonic acid ethyl ester and heated, withstirring for 12 hours at 145° C. while the low-boiling components thatformed were removed by distillation. By fractional distillation of themixture 6.0 g was obtained (yield 81%) of dihydrofuran (b.p.₂.5 mm =120°C.). NMR spectrum (CCl₄, δ): 1.10-1.40 (m, 9 H); 2.14 (s, 3 H); 4.0-4.30(q, 2 H); 4.76 (d, 1 H); 6.50 (d, 1 H).

EXAMPLE 7

10 g of2,4,4-trimethyl-3-carbomethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuranwas dissolved in 200 g of benzene and this solution was passed, drop bydrop, through an upright quartz tube filled with quartz spheres of 0.4cm diameter, in the presence of nitrogen. The quartz tube had a lengthof one meter and a diameter of 3.5 cm. The depth of fill, whichcorresponded to the heating zone, was 30 cm. In the meantime the quartztube was heated externally such that an internal temperature of 350° C.was measured in the center of the heating zone). The dropping rateamounted to 40 grams of solution per hour. The gaseous thermolyzateemerging from the tube was captured in a receiver and cooled to roomtemperature. After a fractional vacuum distillation of the condensate auniform fraction is obtained (b.p.₁.0 Torr : 91°-92° C.) in a yield of7.9 g (=79%), which was identified by NMR spectroscopy and massspectroscopy as2,5,5-trimethyl-3-carbomethoxy-4-(β,β-dichlorovinyl)-4,5-dihydrofuran.

NMR spectrum (100 MHz, CCl4); δ=5.72 (d, 1 H); 3.72 (dq, 1 H); 3.65 (s,3 H); 2.16 (d, 3 H); 1.41 (s, 3 H); 1.31 (s, 3 H).

EXAMPLE 8

10 g of2,4,4-trimethyl-3-carboethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran wasthermolyzed as in Example 7. 7.3 g (=73%) of2,5,5-trimethyl-3-carboethoxy-4-(β,β-dichlorovinyl)-4,5-dihydrofuran(b.p. at 1.5 Torr: 102°-105° C. NMR spectrum (100 MHz, CDCl₃): δ=5.74(d, 1 H); 4.14 (m, 2 H); 3.78 (bd, 1 H); 2.10 (bs, 3 H); 1.48-1.10 (m, 9H).

EXAMPLE 92,4,4-Trimethyl-3-carbethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran

6.1 g of 1,1,1-trichloro-4-methyl-3-pentene-2-ol was mixed with 0.1 g ofAlCl₃ and heated at 145° C. With the exclusion of moisture, 6.1 g of3,3-bisethoxybutyric acid ethyl ester was added drop by drop, withstirring, over a period of 7 hours while maintaining the temperature at145° C. The low-boiling components that formed were distilled out duringthe reaction. After the reaction had ended the mixture was worked up byfractional distillation. The dihydrofuran passed over at a boiling pointof 108°-109° C. at a vacuum of 0.1 mm Hg. The yield is 6.7 g or 80%.

EXAMPLE 10

6.3 g of 1,1,1-trichloro-4-methyl-4-pentene-2-ol was mixed with 6.1 g of3,3-bisethoxybutyric acid ethyl ester and 0.1 g of AlCl₃ and the mixturewas heated for 6 hours at 155° C., while the low-boiling components wereremoved by distillation. The high-boiling components were fractionallydistilled. 6.9 g of 2,4,4-trimethyl-3-carbethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran was obtained, the yield being 83%.

EXAMPLE 112,4,4-Trimethyl-3-carbomethoxy-5-(β,β-dichlorovinyl)-4,5-dihydrofuran

6.1 g of 1,1,1-trichloro-4-methyl-3-pentene-2-ol was mixed with 0.2 g ofAlCl₃ and heated at 150° C. With the exclusion of moisture, 4.8 g of3,3-bismethoxybutyric acid methyl ester was added drop by drop, withstirring, over a period of 4 hours while maintaining the temperature at150° C. At the same time the low-boiling substance that formed wereremoved by distillation. After the reaction had ended the mixture wasfractionally distilled. 6.6 g (yield 83%) of dihydrofuran was obtainedhaving a boiling point of 102° to 104° C. at 0.1 mm Hg).

EXAMPLE 122,4,4-Trimethyl-3-carbethoxy-5-(β,β-dibromovinyl)-4,5-dihydrofuran

10.5 g of 1,1,1-tribromo-4-methyl-3-pentene-2-ol was mixed with 0.1 g ofKHSO₄ and 6.1 g of 3,3-bisethoxybutyric acid ethyl ester and heated withstirring for 12 hours at 145° C., while the low-boiling components thatformed were removed by distillation. By fractional distillation of themixture, 7.7 g (yield 70%) of dihydrofuran was obtained having a boilingpoint of 120° C. at 2.5 mm Hg.

EXAMPLE 13 1-Acetyl-2,2-dimethyl-3-(β,β-dibromvinyl)-cyclopnopanecarboxylic acid-ethylester

Anologues to example A one gram of2,4,4-Trimethyl-3-carbalkoxy-5-(β,β-dibromvinyl)-4,5 dihydrofuran isexposed to ultraviolet light. Purification of the photoproduct occuredwith thin layer chromolographic 0.6 g (60%) of the cyclopropane wasobtained. NMR-spectrum (30 MHz, CCl₄):δ=6.30-6.80 (dd 1 H); 3.70-4.70(m, 2 H); 2.35-2.80 (dd 1 H); 2.30 (n, 3 H); 1.0-1.6 (m, 9 H).

The active insecticide can be synthesized from this ethylester and theother low alkylesters as described in example 1 of this invention.

What is claimed is:
 1. A2,4,4-trimethyl-3-carbalkoxy-5-(β,β-dihalogenvinyl)-4,5-di-hydrofuran ofthe formula ##STR11## wherein R¹ is a straight-chain or branched alkylradical and X represents chlorine or bromine.
 2. A compound according toclaim 1 wherein R¹ is a straight or branched alkyl radical of 1 to 8carbon atoms.
 3. A2,5,5-trimethyl-3-carbalkoxy-4-(β,β-dihalogenvinyl)-4,5-dihydrofuran ofthe formula ##STR12## wherein R¹ is a straight-chain or a branched alkylradical and X represents chlorine or bromine.
 4. A dihydrofuranaccording to claim 3 wherein R¹ is a straight-chain or branched alkylradical of 1 to 8 carbon atoms.